“The circuit diagram is so simple that I didn’t make a complete Eagle schematic for it yet (other builders did, see links). The first prototype was built on a breadboard with round pads. A very similar design could be used for a single-sided printed circuit board. Four bridges (drawn in red colour below) must be soldered on the board before fitting the seven-segment displays. Note that the crystal is mounted on the bottom side ! Some of the display signal are routed with thin enamelled copper wire, especially those between two pads.”